Various toners have been proposed to satisfy a fixing property and a resistance to high-temperature offset of the toner at the same time. In particular, a toner utilizing a binder resin including a polyester component superior in the fixing property and the high-temperature offset resistance, is used in an apparatus requiring a high fixing ability such as a high-speed apparatus. However, such a binder resin including a polyester component leads to various drawbacks in a magnetic toner.
In the case of polyester resin unlike vinyl resin, it is difficult to form a straight chain polymer component with a large molecular weight, it is required to provide the binder resin with a crosslinking structure such as a gel component, and to form a structure capable of avoiding high-temperature offset, which is generated by reduction in the resin viscosity at a high temperature region.
However, such a binder resin having a gel component, when used in a toner, tends to deteriorate a crushing property of the toner. Where crushing is executed under relatively vigorous conditions, the magnetic iron oxide tends to be liberated from the toner particles by an impact in the crushing process. The liberated magnetic iron oxide adversely affects the charging property of the toner, and tends to cause a decrease in the image density or fog. On the other hand, in order to achieve a uniform dispersion under increased adhesion with the binder resin for avoiding the liberation of the magnetic iron oxide, kneading is required to be conducted at high temperature at which the binder resin is sufficiently melted. However, such kneading operation leads to a reduced melt viscosity of the kneaded substance, thus lowering the dispersion of materials other than the magnetic iron oxide and deteriorating the developing property.
Also, the crushing operation under strong impact tends to generate fine toner particles of a very small particle size, called ultra fine powder. Such ultra fine powder deteriorates the charge stability of the toner, thus increasing fog and increasing a sleeve ghost phenomenon.
Such drawbacks become more conspicuous when intending to improve performances such as a higher process speed, a longer service life and a higher image quality by the use of finer toner particles.
As a polymerization catalyst for producing a polyester resin for the toner, a tin-based catalyst has generally been used such as dibutyl tin oxide or dioctyl tin oxide, or an antimony-based catalyst such as antimony trioxide. However, such polymerization catalysts are being investigated further in order to satisfy performance of the magnetic toner suitable for a higher process speed and a higher image quality.
A technology of utilizing a titanic acid ester of an aromatic diol or a solid titanium compound is disclosed as a polymerization catalyst. (For example, see Japanese Patent Application Laid-open No. 2002-148867 and Japanese Patent Application Laid-open No. 2001-64378.)
Also, a technology of employing, as a condensation polymerization catalyst for polyester resin, a titanium tetralkoxide treated with an organic monocarboxylic acid is disclosed. (For example, see Japanese Patent Application Laid-open No. H05-279465.)
However, mere polymerization of a polyester component with such a titanium compound as a polymerization catalyst is not sufficient for improving the adhesion between the magnetic iron oxide and the binder resin.
Also in a one-component developing method with a magnetic toner, advantageously used in the development of electrophotography, a magnetic property and a charging property of the magnetic toner significantly influences the image quality. Particularly in a case where employing a binder resin containing a polyester component, it is necessary to comprehensively control the dispersibility of magnetic iron oxide and the magnetic property of the magnetic toner.
As for the magnetic property of toner, various technologies have been disclosed. (See, for example, Japanese Registered Patent No. 3033614; Japanese Patent Application Laid-open No. H066-317928; Japanese Patent Application Laid-open No. H09-090670; Japanese Patent Application Laid-open No. H09-146297; Japanese Patent Application Laid-open No. H10-171150; Japanese Patent Application Laid-open No. 2002-214829; and Japanese Patent Publication No. S63-043740.)
However, the correlation among the polymerization catalyst for the polyester component, the magnetic property of the toner and the magnetic iron oxide has not been investigated sufficiently and there is a room for improvement.
For the purpose of improving a low-temperature fixing property and a fixing property in a wide fixing temperature range, it is proposed to provide Fischer-Tropsch wax with a polar group thereby achieving improvements in a low-temperature fixing property, a fixing strength and a mutual solubility with the binder resin (for example, see Japanese Patent Application Laid-open No. H09-179342). It is also proposed to include a certain alcoholic wax to secure an appropriate mutual solubility with the binder resin and a wide fixing temperature range (for example, Japanese Patent Application Laid-open No. 2000-338707; and Japanese Patent Application Laid-open No. 2004-029160). However, a relationship between the binder resin and the polar group of wax has not been investigated in consideration of the environmental stability, in addition to the excellent fixing property.
A technology is disclosed for obtaining a toner shape close to a spherical shape by a manufacturing method such as spray granulation, dissolution in a solution or polymerization. (See, for example, Japanese Patent Application Laid-open No. H03-084558; Patent Literature 15: Japanese Patent Application Laid-open No. H03-229268; Japanese Patent Application Laid-open No. H04-001766 and Japanese Patent Application Laid-open No. H04-102862.)
A technology is disclosed for modifying the shape and the surface property of particles by thermal or mechanical impact on the toner produced by a crushing method. (See, for example, Japanese Patent Application Laid-open No. H02-087157; Japanese Patent Application Laid-open No. H10-097095; Japanese Patent Application Laid-open No. H11-149176; and Patent Literature 21: Japanese Patent Application Laid-open No. H11-202557.)
However, with mere modification of the toner shape by such methods, it is difficult to obtain a magnetic toner utilizing a polyester resin capable of providing high image quality.